The original ratings as created by the MPAA in 1968 were G, M, R, X. M was later renamed GP, then PG.
G - all audiences
M - mature (later GP, then PG) - mature content, parental guidance suggested
R - restricted (under 17 not admitted without a parent or guardian)
X - no one under 17 admitted.
Did you know there was only one X-rated movie that won the Academy Award for Best Picture?
Midnight Cowboy was originally rated X when it premiered in 1969; it was later downgraded to R.
Porn movie studios soon co-opted the X rating in an attempt to legitimize themselves. The MPAA later replaced X with NC-17; the first film to receive the rating was
Henry and June.
The PG ratings were later enhanced by a stronger PG-13 rating (parents strongly cautioned; may not be suitable for children under 13).
Red Dawn was the first film rated PG-13.
Back when I was a wee young lad in Venezuela, their movie ratings were quite straightforward: A, B, C, D. (MPAA equivalents in parentheses.)
A = all audiences (G, PG)
B = no one under 14 admitted (PG-13, 'soft' R)
C = no one under 18 admitted (R)
D = no one under 21 admitted (a very strong R to NC-17)
Interestingly, the ratings were controlled by the municipality: it was not unusual for some theaters located in different parts of the city, one would be A, another would be B.
More on the MPAA ratings:
http://www.mpaa.org/movieratings/about/index.htm
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